[issue29335] Python 2.7 subprocess module does not check WIFSTOPPED on SIGCHLD

2017-01-20 Thread Zach Riggle 

New submission from Zach Riggle:

The attached script hits some "This should never happen" code in the subprocess 
module.

These lines here:
https://github.com/python/cpython/blob/2.7/Lib/subprocess.py#L1036-L1038

The root cause is a lack of checking WIFSTOPPED and WSTOPSIG in the handler.

When a process elects into being ptraced via PTRACE_TRACEME, it is stopped on 
the SIGSEGV instead of terminating, allowing the user to attach a debugger 
before the kernel destroys the process.

This bug makes it impossible to wait on any process which crashes, which is set 
up to wait for a debugger.

--
components: Library (Lib)
files: bug.py
messages: 285921
nosy: Zach Riggle
priority: normal
severity: normal
status: open
title: Python 2.7 subprocess module does not check WIFSTOPPED on SIGCHLD
versions: Python 2.7
Added file: http://bugs.python.org/file46363/bug.py

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[issue29335] Python 2.7 subprocess module does not check WIFSTOPPED on SIGCHLD

2017-01-21 Thread Zach Riggle 

Zach Riggle added the comment:

To further clarify the report:

When the attached proof-of-concept is executed, a RuntimeException is raised, 
which has a comment "Should never happen".

The issue isn't due to SIGCHLD, but rather following a waitpid() call.  The 
code attempts to suss the exit code / reason for waitpid() returning, but does 
not check for WIFSTOPPED in its handler.

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[issue29335] subprocess module does not check WIFSTOPPED on SIGCHLD

2017-01-23 Thread Zach Riggle 

Zach Riggle added the comment:

Of note, there's no need to actually cause a SIGSEGV to generate the signal.

The tests might be more clear to replace:

libc.printf(ctypes.c_char_p(0xdeadbeef))

with

os.kill(os.getpid(), signal.SIGSEGV)

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[issue29335] subprocess module does not check WIFSTOPPED on SIGCHLD

2017-01-23 Thread Zach Riggle 

Zach Riggle added the comment:

Neat, though that's not in the standard library.

The current logic for getting a handle to libc could also be simplified via 
ctypes.util.find_library 
(https://docs.python.org/3/library/ctypes.html#finding-shared-libraries).

Darwin:

>>> import ctypes.util
>>> ctypes.util.find_library('c')
'/usr/lib/libc.dylib'

Linux:

>>> import ctypes.util
>>> ctypes.util.find_library('c')
'libc.so.6'

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